Automatic tank level gauges are used to measure the product liquid level in bulk storage tanks. Most level gauges are installed on the tank roof for this purpose. As many of these tanks contains volatile products, where often the vapors are environmentally harmful or even toxic, it is desirable to limit the evaporative losses/emissions. As a result tanks containing these type of harmful products (e.g., gasoline or oil) include breather valves (also known as pressure/vacuum relief valves) which have an open and a closed position so that they are not freely breathing when closed, compared to a vent that would always be open. Opening the breather valve at a pressure protecting level prevents the buildup of excessive pressure or vacuum which can unbalance the system or damage the storage vessel.
Tanks which are freely venting have as a disadvantage that during the day-night cycle they allow product vapors to escape into the atmosphere which can create an environmental hazard, bad smell, and loss of product. Breather valves reduce these losses by keeping the tank closed. During loading product is moved into the tank, and equivalent amount of air is replaced. If this air would not be allowed to leave the tank, it would damage the tank construction and as result it's integrity. If product is moved out of the tank, and no air would be allowed to be replaced needed to remove liquid, so that one would create a ‘vacuum’ and also damage the tank. Hence breathers valve prevent both the build-up of an excessive high and also low pressure. They generally only allow a small over or under pressure, typically several inches of water only (compared to the atmospheric pressure) under the tank roof.
The breather valves can open when excessive pressure occurs, for example when product is pumped into the tank, or can also allow to air enter the tank, when product is pumped out of the tank and a vacuum is created. These breather valves are not servo-controlled, but typically just react on the pressure. This maximum and minimum pressure is normally in the range of 20 mm to 30 mm water pressure. This range although modest is important to understand the function and construction of a typical fixed tank roof and realize the generally enormous size of typical tanks used for bulk storage. These tanks which can easily have diameters of 20 m or more, and experience a considerable ‘lifting’ force when this small pressure is present therein. Accordingly, the tank construction is sensitive for especially the outward (positive) pressure.
Consequently the ‘fixed’ roof on these storage tanks is not firmly welded to the tank construction. Instead, the roof s only welded on the rim, and not on the roof support so that it lays on support beams. As result the roof construction is a not as rigid as one might expect, so that even the relative small vapor pressure caused by the evaporated product in the tank can cause the roof to be lifted considerably, and in the case of an tank fire or explosion, the tank roof can be blown off, since this is safer than if the tank floor fails and the tank explodes into the air. The vapor pressure in the tank can thus affect the level measurement.
An issue that typically arises for level gauges used for inventory and custody transfer applications being tank top mounted and installed high on the tank, is that they can only measure the distance between the level gauge (or more precisely the vertical location where the gauge is installed) and the liquid's top surface. This distance is generally called Ullage or Outage (US). In the level gauge the ‘ullage’ is then converted (mathematically in software) to a product height which tends to be called ‘Innage’. The math is relatively simple: Innage=GRH−Ullage, where GRH=the Gauge Reference Height.
The innage calculation assumes the GRH is stable and doesn't move up and down. It should be clear that if the level gauge is installed on a stilling well or stand pipe then there is provided a stable platform for level gauging. If there is however no stilling well (basically a perforated stand pipe of roughly 6 to 12 inches in diameter) one has a challenge. Installing a stand pipe requires the tank to be out of service, which thus incurs significant cost. Moreover, if the tank is also equipped with a floating roof, it is nearly impossible to retrofit such a tank with a stand pipe at a later stage. Hence in these cases the level gauge is often installed on just the tank roof, and as result the pressure can affect the (innage) level reading.